Soil containing toxic organic chemicals such as chlorinated aromatic hydrocarbons and organophosphorus and organo-nitrogen compounds must sometimes be decontaminated to safeguard water supplies and public health. Remediation is occasionally required, for example, for agricultural pesticides spilled accidentally or over the course of time from improper handling practices. Large-scale spills occur infrequently but require immediate and serious attention. Soil contaminated by repeated contact with concentrates, batch mixes or rinsates is a more insidious problem because applicators often carry out mixing and equipment filling and rinsing operations at the same site on farms and at agrochemical dealerships year after year. Where the pesticides are not only toxic but refractory or persistent, the hazards are compounded.
Research on soil decontamination has focused on bioremediation and physical removal. Many compounds that are significant subsurface soil contaminants such as benzene, toluene, xylenes and alkylbenzenes that enter the ground from gasoline or solvent spills, components of diesel or heating oil such as naphthalane, fluorene, dibenzofuran and other polynuclear aromatic compounds, and halogenated compounds such as chlorobenzene, chlorophenols, and methylene chloride can be biodegraded under aerobic conditions (see, for example, Pardieck, D. L., et al., J. Contam. Hydrol. 9: 221-242 (1992)). Bioremediation is inhibited at high concentrations characteristic of spills, however, and the technique can be limited by survivability of the inocula and pollutant bioavailablity in long-contaminated environments. Moreover, the availability of oxygen for aerobic metabolism is limited in many contaminated soils due to the relatively low solubility of oxygen in groundwater, the slow rate of re-aeration of ground water in the saturated zone, and the significant biological oxygen demand exerted by contaminants in the subsurface (id. at page 222). In addition, the transformation of some important pollutants such as low-molecular-weight halogenated aliphatics and certain pesticides appear to be favored by anaerobic rather than aerobic conditions (ibid.).
From a practical standpoint, physical removal of semi-volatile or non-volatile compounds in the vadose zone is limited to excavation followed by soil washing. Recovered pollutants must be disposed of or destroyed. Many of the compounds that do not degrade pose a threat to biota and/or human populations.
Chemical treatment using non-polluting reagents offers advantages over bioremediation or physical removal. Chemical oxidation by ozone or hydrogen peroxide has been suggested for wastewater decontamination (see, for example, U.S. Pat. No. 5,043,000 to Cater, et al., and U.S. Pat. No. 5,232,484 to Pignatello), and ozone has been suggested to oxidize certain chlorinated organic compounds in the presence of soil or dissolved humic acid (Masten, S. J., Ozone Sci. Engineer. 13: 287-312 (1991)). In soil, however, consumption and/or decomposition of oxidant by soil components, especially soil organic matter, can make complete oxidation uneconomical. Nevertheless, removal of the parent contaminant may be sufficient if byproducts are less toxic or more readlily degraded by indigenous soil microbes.
Fenton-type systems employing ferrous salts and hydrogen peroxide in acidified soil suspensions (pH .about.2 to 3) have been studied as potential oxidants of soil contaminants (Tyre, B. W., et al., J. Environ. Qual. 20: 832-838 (1991) and Leung, S. W., J. Environ. Qual. 21: 377-381 (1992)). For example, pentachlorophenol and trifluralin were shown to be extensively degraded in soil suspensions when treated with .about.4.times.10.sup.-3 M ferrous ion and 3.5 M (120 g/L) hydrogen peroxide; hexadecane and dieldrin were partially transformed under similar conditions (Tyre, et al., cited above). Tetrachloroethene was mineralized in silica sand suspensions treated with 5.times.10.sup.-3 M ferrous ion and 2.1 M hydrogen peroxide (Leung, et al., cited above).
In the classic Fenton reaction, ferrous ion rapidly reduces hydrogen peroxide to an hydroxyl radical. A disadvantage of using ferrous ion is that it is required in stoichiometric amounts. Peroxide demand and therefore ferrous ion demand can be high due to competitive oxidation of soil organic matter and soil-catalyzed decomposition. A further disadvantage of ferrous ion is that it is oxidized by hydroxyl radicals and therefore it competes with the target compounds unless its concentration is kept low by gradual addition in dilute form.
Ferric ion may also produce hydroxyl radicals from peroxide, albeit at a slower rate than ferrous ion. However, the use of ferric ion requires acidic conditions to keep the iron soluble; the reaction has a pH optimum of about 3. In soil decontamination, the need to acidify the soil is a serious drawback to use of ferric ion. Acidification to an optimum pH of 3 is difficult and clumsy because soil has a high buffering capacity. Moreover, acidification itself can be viewed as a polluting practice unless the soil were excavated for treatment and neutralized before replacement.